Metallic powder having average particle diameters of 1 μm or less (which may be called “ultrafine” particles) such as Ni, Ag, Cu, or Fe are very useful as a conductive paste for electrical materials, in particular, for internal electrodes forming material or magnetic material used in multilayer ceramic capacitors.
Heretofore, noble metallic powders such as Ag, Pd, Pt and Au or base metallic powders such as Ni, Co, Fe, Mo and W are used in conductive pastes for electrical materials, in particular, as conductive pastes forming internal electrodes for multilayer ceramic capacitors. Generally, a multilayer ceramic capacitor is made by alternately laminating ceramic dielectric layers and metallic layers which are used as internal electrodes, and by forming external electrodes which are connected to the metallic layer of internal electrodes on both outsides of the ceramic dielectric layers. Material having a high dielectric constant material as a main component such as barium titanate, strontium titanate and yttrium oxide are used in the dielectric substances. On the other hand, noble metallic powders or base metallic powders mentioned above are used as metal which forms the internal electrodes. However, since more inexpensive electrical materials are required recently, the latter base metallic powders are considered to be more useful. In particular, a multilayer ceramic capacitor in which electrodes are made by forming ceramic dielectric layers of ultrafine nickel layers having particle diameters of 0.1 to 1.0 μm have been greatly developed.
Ultrafine metallic powders described above can be produced by various methods. Recently, vapor phase reduction, in which metal chloride vapor and reducing gas such as hydrogen are contacted, is widely adopted. By this method, ultrafine metallic powder having diameters of 1 μm or less can be obtained inexpensively, and the diameter of particles can be freely controlled.
However, metal chloride and hydrochloric acid may remain on the surface of the ultrafine metallic powder when metal chloride is used as a raw material. These chloride components are difficult to remove by washing with water. Recently, the size of multilayer ceramic capacitors has been reduced and the capacitance of the multilayer ceramic capacitors has increased, and not only metallic powders having average diameters of 0.4 μm but also metallic powders having average diameters of 0.1 to 0.2 μm are required. However, the smaller the particle diameters become, the greater the chloride components contained in metallic powder produced in a reducing furnace become, and the chloride components are difficult to remove.
These chloride components which are contained in metallic powder reduce the purity of the ultrafine metallic powder as product, promote oxidation of the metal, and cause the deterioration (rust) of the metal. Furthermore, the chloride components cause deterioration over time of the conducting paste, and they may influence the characteristics of the electrode formed by the paste. Therefore, ultrafine metallic powder which has less chloride components and higher purity is required as a material for electrodes used in, for example, multilayered ceramic capacitors.
To remove chloride components contained in metallic power, washing process with water can be considered. However, the sedimentation rate of metallic powder in water becomes slower as chloride components are reduced by this water-washing, it becomes difficult to separate and collect the metallic powder by decantation, and as a result, washing efficiency is reduced. Furthermore, yield of the product may be reduced by removing supernatant which contains metallic powder by decantation. Therefore, development of a technique to remove chloride components has become important.
To remove chloride components contained in metallic powder such as nickel powder, a technique in which metallic powder is washed with water which contains organic acid is disclosed in Japanese Unexamined Patent Application Publication No. 189813/99, and a technique in which metallic powder is washed with water which contains chelating agent is disclosed in Japanese Unexamined Patent Application Publication No. 346119/94. Although these techniques can remove the chloride component sufficiently, metallic powders aggregate together to form coarse particles, or precipitates may adhere. In a forming process for a membrane electrode having a thickness of about 1 to 2 μm, a non-uniform paste coating may be formed and a multilayered electrode membrane cannot be formed any longer if these coarse particles and aggregated coarse particles exist. Therefore, a removing process for the coarse particles is required using a method such as classification by a liquid cyclone. However, such coarse particles in which metallic powders are aggregated are difficult to remove. From this viewpoint, the technique described above is insufficient.
There is also another problem in that hydroxides of metals are generated on the surface of the metallic powder by washing the metallic powder in water. Non-uniform oxide layer is formed on the surface of the metallic powder if the metallic powder having hydroxide on its surface is dried. As a result, dispersibility is deteriorated and the metallic powder agglomerates together in the case in which conductive paste is formed. Furthermore, in the case in which the conductive paste is used as internal electrodes in a multilayer ceramic capacitor, cracking or delamination may occur because the sintering characteristics of the metallic powder are unstable.
Therefore, objects of the present invention are to provide a process for production of metallic powder in which chloride components and hydroxide are efficiently removed, chloride components are extremely small, and minimal coarse particles are included as an after-treatment of the metallic powder made by vapor phase reduction which is applied to metal chloride as a raw material, and also to provide the metallic powder produced by this method, and conductive paste and multilayer ceramic capacitors formed by the metallic powder.
Objects of the present invention are described more concretely as follows.    (1) To provide a method in which chloride components contained in metallic powder can be washed and removed.    (2) To provide a washing method in which the metallic powder can precipitate immediately even after chloride components are removed.    (3) To provide a washing method in which chloride components can be removed without organic compounds remaining on the surface of the metallic powder.    (4) To provide a method in which aggregation of the metallic particles does not occur in the washing process and therefore classification can be applied efficiently.    (5) To provide metallic powder having sufficient dispersibility and sintering characteristics by removing hydroxide and forming uniform oxide layer on the surface of the metallic powder.
Although the metallic powder produced in the present invention is very suitable as a raw material of internal electrodes in a multilayer ceramic capacitor, it is not limited to this, and this metallic powder is also suitable for other uses such as for sintered materials, magnetic materials, or catalysts.